Heat exchanger and mixer

ABSTRACT

Mixing apparatus comprising a gang of mixers each having an elongated jacketed hollow shell with an inlet and an outlet for process material and a series of pairs of helical blades mounted in axially adjacent and circumferentially offset relation in the shell with the blades in axially adjacent pairs being pitched oppositely to cause the material to be divided and folded or twisted in alternate directions as it advances through the shell is provided with means to transfer heat through the blades and to the process material. The blades are hollow and are mounted on a hollow tubular shaft which has internal manifold means providing fluid communication between adjacent ends of the pairs of blades. A conduit extends centrally through the shaft and forms an annular axially extending passage adjacent the bases of the blades for containing a heat-transfer fluid and transferring heat therefrom. Heat transfer fluid flowing through the blades is returned to its source at the other end of the shaft through the return conduit and the annular passage. There is provided means to rotate the shaft at a relatively slow speed to enhance the mixing and heat transfer action in the apparatus by sweeping from the inside of the shell relatively stagnant films which otherwise would build up.

United States Fatent [1 Root [451 Oct. 16, 1973 HEAT EXC HANGER AND MIXER [75] Inventor: William L. Root, Bethlehem, Pa.

[73] Assignee: The Bethlehem Corporation,

Bethlehem, Pa.

[22] Filed: Feb. 9, 1972 [21] Appl. No.: 224,677

Primary Examiner-Charles Sukalo Att0rneyDexter N. Shaw et a1.

[57] ABSTRACT Mixing apparatus comprising a gang of mixers each having an elongated jacketed hollow shell with an inlet and an outlet for process material and a series of pairs of helical blades mounted in axially adjacent and circumferentially offset relation in the shell with the blades in axially adjacent pairs being pitched oppositely to cause the material-to be divided and folded or twisted in alternate directions as it advances through the shell is provided with means to transfer heat through the blades and to the process material. The blades are hollow and are mounted on a hollow tubular shaft which has internal manifold means providing fluid communication between adjacent ends of the pairs of blades. A conduit extends centrally through the shaft and forms an annular axially extending passage adjacent the bases of the blades for containing a heat-transfer fluid and transferring heat therefrom. Heat transfer fluid flowing through the blades is returned to its source at the other end of the shaft through the return conduit and the annular passage. There is provided means to rotate the shaft at a relatively slow speed to enhance the mixing and heat transfer action in the apparatus by sweeping from the inside of the shell relatively stagnant films which otherwise would build up.

11 Claims, 5 Drawing Figures SHEET 10F 2 PATENIED "III 16 I975 HEAT EXCHANGER AND MIXER The present invention relates'to mixing apparatus having heat-transfer capabilities, and more particularly, the present invention relates to mixing apparatus of the type having hollow mixing blades through which a heat transfer medium is flowed.

Within the past few years a considerable amount of work has been done in the art of providing axial-flow mixers having a stationary system of blades which act on a flowing medium to split it into a plurality of streams and to fold or twist the split streamsin alternate directions for effecting a mixing action as the medium flows through the mixer. Examples of such mixers are disclosed in U. S. Pat. No. 3,286,992 and in'an article entitled Motionless lnline Mixers Stir Up'Broad Interest at pages 94-96 in the May 19, 1969 issue of the Chemical Engineering magazine. Although the mixers disclosed in the aforementioned publicationsmay operate satisfactorily to mix relatively viscous process material in relatively small (up to 12 inches) diameter tubes,'none is provided with means to transferheat to or from the material.

Heat transfer is often necessary where a mixing action causes undesirable thermal conditions inside the mixer due to exothermic or endothermic reactions. In mixers of relatively large (more than 12 inches) diameter, the presence of thermal gradients in a plane normal to the flow direction of the material being-processed may cause unwanted and possibly deleterious chemical reactions to occur inside the mixer. Thus, a nonhomogenous product may result. In an effort to afford mixers referred to above with heat-transfer capabilities, jackets have been provided around the mixers. Although jacketed in-line mixers afford a certain degree of heat-transfer capabilities, such capabilities become limited as the diameter of the mixer increases about 12 inches.

In my U. S. Pat. Nos. 3,250,321 and 3,785,330, there are disclosed mixing devices having heat-transfer means associated with mixing blades. Although these patented machines operate satisfactorily, a mixer which possesses axial-flow stream-splitting and twisting characteristics along with efficient means to transfer heat relative to the material being processed is highly desirable. Moreover, such a mixer having a jacket containing a heat-transfer medium and means to slowly rotate the mixing blades as the material advances to improve the heat-transfer and mixing characteristics of the mixer is also desirable.

With the foregoing in mind, it is. the primary object of the present invention to provide a novel axial-flow mixer having means to transfer heat-relative to the material being mixed as the material advances through the mixer.

It is another object of the present invention to provide an axial-flow mixer having hollow blades mounted on a hollow shaft and manifold means in the shaft for supplying heat-transfer fluid to the blades and for flowing the fluid axially in the shaft adjacent their bases.

It is a further object of the present invention to provide means for slowly rotating the blades in a jacketed axial-flow mixer while transferring heat through the jacket and the blades to improve the mixing and heattransfer action in the mixer.

More specifically, in the present invention a mixer comprising a jacketed shell having an inlet and an outlet and a series of pairs of helical blades disposed in axially adjacent relation and offset circumferentially around a hollow shaft rotatably mounted in the shell is provided with means to transfer heat through the blades and relative to material being advanced through the shell. The blades in axially adjacent pairs are pitched oppositely to cause the material flowing through the shell to be split and twisted or folded and thereby mixed as it advances, and the blades are hollow to provide passages through which a heat-transfer medium is flowed. A supply fitting is connected atone end of the shaft for introducing the heat-transfer medium to the blades, and manifold means is provided in the shaft to provide fluid communication among the passages in the blades. The manifold means includes a conduit which extends centrally through the shaft and which forms with the shaft an annular passage along the bases of the blades. The heat-transfer medium flows in one direction through the blades to the other end of the shaft where a major portion enters the conduit and flows in the opposite direction to the supply fitting; a minor portion of the medium enters the annular passage and flows therethrough to the supply fitting. There is provided means to rotate the shaft and blades at a relatively slow speed for improving the heat-transfer and mixing'attributes of the apparatus by scraping the inside of the shell to remove any stagnant layers which otherwise would tend to reduce the efficiency of heat transfer inwardly from the jacket.

These and other objects, features and advantages of the'present invention should become apparent from the following description when taken in conjunction with the accompanying drawings in which:

FIG. I is an elevational view of a gang of mixers embodying the present invention, the lower mixer having its shell broken away to expose a mixing blade assembly mounted therein;

FIG. 2 is a diagrammatic view illustrating the path of flow of a heat-transfer medium through a manifold which supplies the medium to the mixing blades;

FIG. 3 is an enlarged longitudinal sectional view of a portion of the blade assembly illustrated in FIG. 1;

FIG. 4 is an enlarged transverse sectional view of the blade assembly taken in axially spaced planes to illustrate passage means for flowing the heat transfer medium through the blades and manifold means for supplying the medium to the passage means; and

FIG. 5 is a sectional view illustrating means for returning to its source heat transfer medium which has flowed through the blades.

Referring now to the drawings, there is illustrated in FIG. 1 axial-flow mixing apparatus 10 having heattransfer capabilities. In the present instance, the apparatus 10 comprises three horizontally-disposed mixers l1, l2 and 13 which are ganged together to provide a sinuous flow path for process material. The mixers have inlets 11d, 12a, and 13a, respectively, and outlets 11b,

12b, and 13b, respectively. The inlet 11a for the topmost mixer in the gang is provided by a flanged lateral pipe which is adapted to.be bolted to a supply pipe (not shown) for feeding process material under pressure to the apparatus 10. The outlet 13b for the lowermost mixer 13 is provided by a flanged downturned elbow which is adapted to be bolted to auxiliary processing equipment (not shown). The inlet 12a and outlet 12b of the intermediate mixer 12 is connected by means of flanged elbows disposed in the form of Us 14 and 15 to the outlet 11b and inlet 13a of the upper and lowermost mixers 11 and 13, respectively. With this structure, process material is flowed downwardly in a serpentine path as it is being processed in the apparatus.

In order to mix the process material thoroughly as it advances through the apparatus, each mixer has a hollow shell, such as the shell 19 of the lower mixer 13, and a series of pairs of mixing blades 21, 22 and 23 such as the blades illustrated in the cutaway portion of the lower mixer 13. An elongated shaft 20 extends centrally and longitudinally in the shell 19, and, as shown, each of the blades spans radially between the shaft 20 and the inner surface of the shell 19 along the entire length of the blade. The blades in the pairs 21, 22 and 23 extend helically and are spaced circumferentially from one another on the shaft 20. The blades in each pair are pitched in the same direction, and the blades in axially-adjacent pairs are pitched in the oppositedirection. As best seen in FIG. 3, one blade in each pair, such as the blade 21a in the first pair 21, is twisted in a right-hand helix as indicated in broken lines to turn through an included angle of l80 between its leading and trailing edges, and the other blade 21b in the first pair 21 is twisted in a right-hand helix as indicated in dashed lines to turn through the same angle. The blades 22a and 22b in the second or axially adjacent pair 22 downstream of the first pair 21 are disposed with their leading edges offset or at right angles with respect to the trailing edges of the first pair 21 and are twisted in a left-hand helix. The blades in the third pair 23 are pitched as are the blades in the first pair 21. Thus, as process material flows through the mixer 13, the pairs of blades 21, 22 and 23 operate to split the flowing material into streams and to twist the split streams in alternate directions to effect a mixing and blending action.

Of course, when mixing certain process materials it may be desirable for the blades to turn through more or less than 180 and for the blades in axially adjacent pairs to be offset at more or less than the noted rightangle. Also, the number of blades, their pitch, and like factors must be'considered in the ultimate design of the apparatus. For a more detailed description of the mechanics of the mixing action, and certain design considerations, reference is made to the aforementioned publications.

In accordance with the present invention, there is provided means to transfer heat relative to the process material as it advances through the apparatus 10. To

this end, each blade, for example, the blade 21a in the first pair 21, comprises a pair of shaped plates 21.,21 (FIG. 4) welded at their bases to the outside of the shaft 20. A scraping member 25 is welded between the plates 21 ,21 and projects outwardly therebeyond into closely-spaced scraping relation with respect to the shell 19 to prevent a buildup of process material on the inside of the shell. Thus, this construction provides each blade with a flow passage 26 which extends radially and axially for permitting a heat-transfer medium, such as hot or cold water, to be flowed through each blade for transferring heat relative to the material being processed through the mixer.

For the purpose of supplying the heat-transfer medium to the blades and for exhausting the medium after it has transferred heat with respect to the process material, fittings31, 32 and 33, respectively, are provided. As best seen in FIG. 1, each fitting, such as the fitting 33 on the lower mixer 13, is coupled to the left end of the shaft 20 and is provided with a tapped radial inlet 33a and a tapped axial outlet 33b. The fitting 33 is similar in construction to the fittings employed on the mixing apparatus disclosed in my aforementioned patents, and particularly in FIGS. 1 and 2A of my U.S. Pat. No. 3,285,330.

The fitting 33 cooperates with manifold means in the shaft 20 to distribute the heat transfer fluid to the blades 21, 22 and 23. To this end, the manifold means comprises a series of pairs of pockets 41, 42 and 43 located at axially spaced intervals in the shaft 20 and arranged in the shaft 20 to underlie the blades 21, 22 and 23 adjacent their leading and trailing edges. As best seen in FIG. 3, the upper and lower pockets 41 and 41"., respectively, in the pair 41 extend in opposite directions axially in the shaft 20 upstream and downstream of the adjacent edges of the first and second pair of blades 21 and 22. In the present instance, the upper pocket 41 provides fluid communication between the rear one 21b of the first pair of blades 21 and the front one 22a of the second pair of blades 22 through exit port 51' and entrance port 52', respectively, in the shaft 20. The trailing and leading edges of these blades are disposed at to one another when viewed in the direction of fiow of process material through the mixer 13. The lower pocket 41" similarly provides fluid communication between the other axially adjacent blades in the pairs by means of exit port 51" and entrance port 52" (FIG. 4). The third pair of blades 23 is similarly connected to the second pair 22 through the pair of pockets 43, exit ports 52" and 52", and entrance ports 53' and 53" (see FIG. 2).

The heat-transfer medium is flowed in one direction through the blades 21, 22 and 23 and the others and'is returned in the opposite direction through the shaft 20. To this end, a conduit 28 (FIG. 3) is mounted centrally within the shaft 20 and extends longitudinally therethrough to define an annular passage 29 between the shaft 20 and the conduit 28. The conduit 28 is supported at spaced intervals along its length by means of baffles such as 30,30 which extend between the conduit 28 and the inside of the shaft 20- to form the pairs of pockets, such as the pockets 41' and 41". In order to introduce the heat transfer medium to the blades, a hollow plug 25 is provided in the shaft 20 at the end adjacent the fitting 33, the plug 25 having a boss 25a engaging inside the conduit 28 to support the conduit 28 centrally in the shaft 20 and having an axially extending tube 25b opening into the conduit 28. The heat transfer medium is flowed from the fitting 33 into the plug 25 and outwardly from an annular chamber 25c surrounding the tube 25b in the plug 25 and into the first pair of blades 21a and 21b through ports 54a and 54b, respectively, in the plug 25 and shaft 20.

For the purpose of returning to the supply fitting 33 the heat-transfer medium flowing through the blades, a plug 27 is provided in the other end of the shaft 20. As best seen in FIG. 5, the plug 27 has a boss 27a which engages inside the conduit 28 to support the same centrally in the shaft 20, and a pair of ports 55a and 55b are provided in the shaft 20 adjacent the plug 27 to permit the heat-transfer medium to flow inwardly from the blades and into the annular passage 29 around the conduit 28. A major portion of the heat-transfer medium entering the passage 29 adjacent the plug 27 is admitted into the conduit 28 by means of ports or apertures 28a,28a and is flowed axially through the conduit 28 to the plug 25 at the other end of the shaft 20 and through the tube 25b in the plug 25 to the return portion of the fitting 33. A minor portion of the heat-transfer medium flowing into the shaft from the blades is flowed axially through the annular passage 29 in the shaft and is admitted into the conduit 28 by means of ports or apertures 28b,28b for flowing through the tube b and the plug 25 and to the fitting 33. Preferably, the ports 28a and 28b are sized in such a manner as to ensure that a major portion of the heat-transfer medium flowing through the blades returns through the conduit 28.

Although a substantial amount of heat transfer occurs between the blades and the process material, some heat transfer takes place from the shaft 20 and to the process material by virtue of the flow of returning medium through the annular passage 29 in the shaft 20. In addition to these heat transfer mechanisms, heat is transferred inwardly from the periphery of the shells of the mixers, such as the shell 19 of the mixer 13. For this purpose, as best seen in FIG. 4, a jacket 18 surrounds the shell 19 and defines an annular passage 17 along the length of the mixer 13. An upstanding inlet connection 16a is provided in the jacket 18 at one end of the mixer 13 and a depending outlet connection 16b isprovided in the jacket 18 at the other end of the mixer 13 to permit a heat-transfer fluid to be flowedthrough the passage 17. The medium flowing into the passage 17 is guided longitudinally in the passage 17 by means of a series of vanes 17a,17a spaced apart peripherally in the passage 17 and extending longitudinally of the mixer 13. With this structure, a heat-transfer medium flowing through the passage 17 from the inlet connection 16a to the outlet connection 16b transfers heat inwardly through the shell 19 of the mixer 13 (and similarly in the other mixers 11 and 12) to the process material advancing through the mixer. Of course, the outside of the jacket 18 is suitably insulated to minimize heat transfer radially outward from the passage 17.

For the purpose of promoting the exchange of heat between the medium in the jacket-passage l7 and the process material, there is provided means to rotate the series of blades at a relatively slow speed. As best seen in FIG. I, the blade rotation means includes a motor and speed reducer assembly 45 connected to the shaft 20 through a suitable coupling 46 and pillow block 47. A similar pillow block 48 is provided to rotatably mount the other end of the shaft 20 adjacent the supply and exhaust fitting 33. The ends of the shaft 20 are coupled in a fluid-tight manner to the shell by means of packing glands, stuffing boxes or like connections 49a and 49b. Thus, the blades in each mixer are capable of being rotated at a relatively slow speed, for instance, in a range of about 2-4 rpm, to scrape from the interior of the shell 19 stagnant layers of process material which otherwise would build up and tend to reduce the efficiency of heat transfer. It is believed that a continuous shearing or scraping action is effected between the blades and the interior of the shell 19 and that the scraping action causes the process material adjacent the inside wall of the shell 19 to be displaced radially inward across the faces of the blades, thereby removing stagnant layers of process material from the blades and to improve their heat-transfer efficiency.

Certain advantages are realized when the mixers are ganged together as illustrated in FIG. 1, for example, in

the third or uppermost mixer 11 and the first or lowermost mixer 13, the flow of process material is leftward,

and the flow of heat-transfer medium through the mixing blades is rightward or counter to the flow of the process material. Hence, even though at start-up, the temperature of the heat-transfer medium (assuming the material is being heated as it is mixed) at the inlets to those mixers may be lower than at the outlets, in the steady state the effect of such temperature gradient is minimized, because the process material is being continuously heated as it flows leftward. In the center or secondmixer 12, of course, the heat-transfer medium flows through the blades in the same direction as the process material flows through the mixer. As a result of the gang of mixers, the effect of any temperature gradient which may exist along the length of any one of the mixers is minimized.

In view of the foregoing, it should be apparent that improved axial-flow mixing apparatus has now been provided with a capability of transferring heat relative to material being processed through the apparatus. The description of the heat transfer has been in terms of transfer from the heat exchange medium to the process material. Obviously the transfer may alternatively be from the process material to the heat exchange medium, according to the temperature differential which exist.

While a preferred embodiment of the present invention has been described in detail, various modifications, alterations and changes may be made without departing from the spirit and scope of the present invention as defined in the appended claims.

I claim:

l. A heat exchanger and mixer comprising a hollow shell having an inlet'for admitting process material and an outlet spaced from the inlet for discharging the process material, a shaft extending through said shell, a series of helical blades arranged axially adjacent one another on said shaft and extending outwardly into the proximity with said shell, the trailing end of one blade being offset circumferentially from the leading end of the adjacent blade, said adjacent blades having pitches alternating in opposite directions in the path of movement of the process material to divide the process material and to fold the material in alternating directions to effect a mixing action as the process material advances forwardly through the mixer from the inlet to the outlet, each blade having a helical flow passage extending outwardly from said shaft and between said leading and trailing ends, means to flow a heat-transfer medium through said blades for transferring heat relative to said process material including manifold means in said shaft providing fluid communication among the flow passages in said blades, said manifold means including port means spaced apart in said shaft adjacent the leading and trailing edges of said blades for providing fluid communication with the interior of said shaft so that the heat-transfer medium may be flowed into each blade at one location and out of each blade at an axially spaced location after flowing through the length of the blade.

2. Apparatus according to claim 1 wherein each blade includes a pair of shaped plates spaced from one another to provide said blade-passage and a scraping member sandwiched between said plates along their outer edges and extending radially outward into proximity with said shell.

3. Apparatus according to claim 1 including means connected to said shaft to displace said shaft and said blades in said shell to effect a scraping action of said medium, means providing an inlet connection to said second passage and an outlet connection from said second passage for enabling said medium to flow therethrough, and a series of vanes in said second passage for guiding said medium longitudinally in said second passage from said inlet connection to said outlet connection, said shaft-displacing means including means to rotate said shaft, whereby heat may be transferred radially relative to process materials advancing through the mixer.

5. Apparatus according to claim 1 including means mounted at one end of said shaft for supplying said heat-transfer medium to said manifold means in said shaft and for collecting said medium from said manifold means after it has transferred heat relative to the process material, means mounted at the other end of said shaft for receiving heat-transfer medium from the blades in said series and for returning s'aid medium through said shaft to said supply means, whereby the heat-transfer medium flows in one direction through the blades and is returned in an opposite direction through the shaft.

6. Apparatus according to claim 5 wherein said manifold means includes a conduit mounted within said shaft,and a series of pockets provides between said conduit and said shaft, said port means including a pair of ports located at opposite ends of said blades in registry with said pockets, whereby the heat-transfer medium flows through the blades and their associated pockets from the supply means to the returning means.

7. Apparatus according to claim 6 wherein said blades are arranged in pairs on said shaft with the blades in each pair having diametrically extending leadthird one of said mixers being connected to the inlet 'of said second mixer and the outlet of said second mixer being connected to the inlet of said first mixer, said heat-transfer medium supply means for said mixers being grouped together at one end of each mixer with said outlet of said third one of said mixers being located adjacent its associated heat-transfer medium supply means, 'whereby the heat-transfer medium flows through the blades in a direction counter to the flow of process material in said first and third mixers and flows through said blades in a direction with said process material in said second mixer.

9. Apparatus according to claim 5 including a return conduit mounted centrally in said shaft for flowing said return heat-transfer medium to said supply means, said return conduit cooperating with said shaft to define an annular passage along the bases of the blades for flowing said heat-transfer medium through said shaft, said receiving and returning means at said other end of said shaft including return ports providing fluid communication between saidblade-passages and said annular passage and portrneans providing fluid communication between said annular passage and the interior of said conduit adjacent each end of said conduit to permit a portion of the heat-transfer medium exiting from said blades at the other end of the shaft to flow through the annular passage and another portion to flow through the c0nduit.'-

10. Apparatus according to claim 9 wherein said port means in said conduit include apertures in said conduit, said apertures being sized to cause the portion of heattransfer medium flowing through the conduit to be a major portion of the total flow through the blades.

11. Apparatus according to claim 9 wherein said heat-transfer medium supply means includes a hollow plug in said one end of said shaft, a tube extending axially in said plug and in fluid communication with the interior of said conduit for flowing through said plug heat-transfer medium returning from said blades, said tube forming with said plug an annular chamber therein, and including supply ports providing fluid communication between said annular chamber and said blade passages, whereby the heat-transfer medium is flowed radially outward from the plug and into the blades and is returned through the conduit from the blades. 

1. A heat exchanger and mixer comprising a hollow shell having an inlet for admitting process material and an outlet spaced from the inlet for discharging the process material, a shaft extending through said shell, a series of helical blades arranged axially adjacent one another on said shaft and extending outwardly into proximity with said shell, the trailing end of one blade being offset circumferentially from the leading end of the adjacent blade, said adjacent blades having pitches alternating in opposite directions in the path of movement of the process material to divide the process material and to fold the material in alternating directions to effect a mixing action as the process material advances forwardly through the mixer from the inlet to the outlet, each blade having a helical flow passage extending outwardly from said shaft and between said leading and trailing ends, means to flow a heat-transfer medium through said blades for transferring heat relative to said process material including manifold means in said shaft providing fluid communication among the flow passages in said blades, said manifold means including port means spaced apart in said shaft adjacent the leading and trailing edges of said blades for providing fluid communication with the interior of said shaft so that the heat-transfer medium may be flowed into each blade at one location and out of each blade at an axially spaced location after flowing through the length of the blade.
 2. Apparatus according to claim 1 wherein each blade includes a pair of shaped plates spaced from one another to provide said blade-passage and a scraping member sandwiched between said plates along their outer edges and extending radially outward into proximity with said shell.
 3. Apparatus according to claim 1 including means connected to said shaft to displace said shaft and said blades in said shell to effect a scraping action of said blades along the interior of said shell.
 4. Apparatus according to claim 3 including a jacket surrounding said shell and cooperating with said shell to form a second passage for containing a heat-transfer medium, means providing an inlet connection to said second passage and an outlet connection from said second passage for enabling said medium to flow therethrough, and a series of vanes in said second passage for guiding said medium longitudinally in said second passage from said inlet connection to said outlet connection, said shaft-displacing means including means to rotate said shaft, whereby heat may be transferred radially relative to process materials advancing through the mixer.
 5. Apparatus according to claim 1 including means mounted at one end of said shaft for supplying said heat-transfer medium to said manifold means in said shaft and for collecting said medium from said manifold means after it has transferred heat relative to the process material, means mounted at the other end of said shaft for receiving heat-transfer medium from the blades in said series and for returning said medium through said shaft to said supply means, whereby the heat-transfer medium flows in one direction through the blades and is returned in an opposite direction through the shaft.
 6. Apparatus according to claim 5 wherein said manifold means includes a conduit mounted within said shaft, and a series of pockets provided between said conduit and said shaft, said port means including a pair of ports located at opposite ends of said blades in registry with said pockets, whereby the heat-transfer medium flows through the blades and their associated pockets from the supply means to the returning means.
 7. Apparatus according to claim 6 wherein said blades are arranged in pairs on said shaft with the blades in each pair having diametrically extending leading and trailing edges, said pockets and ports being arranged in pairs at said locations with one pocket in each pair providing fluid communication between adjacent offset blades in said axially adjacent pairs.
 8. Apparatus according to claim 5 including second and third like mixers ganged with the first-mentioned mixer, each mixer having an inlet and an outlet and heat-transfer medium supply means, the outlet of the third one of said mixers being connected to the inlet of said second mixer and the outlet of said second mixer being connected to the inlet of said first mixer, said heat-transfer medium supply means for said mixers being grouped together at one end of each mixer with said outlet of said third one of said mixers being located adjacent its associated heat-transfer medium supply means, whereby the heat-transfer medium flows through the blades in a direction counter to the flow of process material in said first and third mixers and flows through said blades in a direction with said process material in said second mixer.
 9. Apparatus according to claim 5 including a return conduit mounted centrally in said shaft for flowing said return heat-transfer medium to said supply means, said return conduit cooperating with said shaft to define an annular passage along the bases of the blades for flowing said heat-transfer medium through said shaft, said receiving and returning means at said other end of said shaft including return ports providing fluid communication between said blade-passages and said annular passage and port means providing fluid communication between said annular passage and the interior of said conduit adjacent each end of said conduit to permit a portion of the heat-transfer medium exiting from said blades at the other end of the shaft to flow through the annular passage and another portion to flow through the conduit.
 10. Apparatus according to claim 9 wherein said port means in said conduit include apertures in said conduit, said apertures being sized to cause the portion of heat-transfer medium flowing through the conduit to be a major portion of the total flow through the blades.
 11. Apparatus according to claim 9 wherein said heat-transfer medium supply means includes a hollow plug in said one end of said shaft, a tube extending axially in said plug and in fluid communication with the interior of said conduit for flowing through said plug heat-transfer medium returning from said blades, said tube forming with said plug an annular chamber therein, and including supply ports providing fluid communication between said annular chamber and said blade passages, whereby the heat-transfer medium is flowed radially outward from the plug and into the blades and is returned through the conduit from the blades. 